In a push-pull converter having a center-tapped transformer and a pair of power switches which are connected to the center tapped primary transformer winding, the switches are controlled to alternately conduct, thereby developing an AC output in the secondary winding of the transformer. The AC output from the secondary winding of the transformer is rectified and filtered to provide a regulated DC output voltage. The regulated DC output voltage is applied to a feedback control circuit which detects errors in the DC output and controls the power switches to maintain the DC output at a desired level. The feedback control circuit typically includes an error amplifier which compares the regulated DC output to a fixed reference. The output of the error amplifier is then applied to a pulse width modulator which compares the error signal to a ramp signal to provide a pulse width modulated waveform having a duty cycle which varies with the error signal, the pulse width modulated waveform being used to control the power switches to regulate the DC output.
In order to prevent large inrush currents during the start-up of the system, soft-start controls have been employed. Known soft-start controls such as shown in Koizumi et al U.S. Pat. Nos. 4,180,852 and Saleh 4,353,114 for a DC to DC converter have been connected to the output of the error amplifier so that the duty cycle of the pulses generated by the pulse generator gradually increases when the converter is first turned on. Such known soft-start controls typically include a capacitor connected across the output of the error amplifier, the capacitor being charged by a constant current source. The soft-start capacitor holds the output of the error amplifier so that it does not go to a high level immediately after the system has been turned on, but slowly increases to the desired level.
The soft-start duration for such controls is typically long in order to bring the inrush currents down to acceptable levels. However, a long soft-start duration is not desirable for many applications. When the soft-start duration is large, a flux imbalance in the transformer may result which can cause saturation of the transformer core resulting in failure of the system. Known flux balancing circuits which provide a correction signal to vary the output of the error amplifier are very effective during the steady-state operation of the converter to bring the transformer into a balanced condition. However, during the start-up of the converter, the known soft-start circuits, connected to the output of the error amplifier, override the operation of the flux balancing circuit which is also connected to the output of the error amplifier. Therefore, flux balancing circuits have not been effective during start-up to prevent flux imbalances from causing system failures.